The invention relates to a process for purification of a feedstock that contains carbon dioxide and gaseous and liquid hydrocarbons that are recovered from at least one production well operating with an injection of carbon dioxide.
The technological background is illustrated by U.S. Pat. Nos. 4,333,529, 3,065,790 and 3,150,716.
The assisted recovery of petroleum is commonly used in the petroleum industry to recover crude oil that remains in place in a formation after its natural or forced production or to recover heavy fuel that is too viscous to flow naturally or artificially using unsophisticated pumping means.
To date, steam is the driving force that is most commonly used to flush crude oil formations effectively, but production programs are increasingly implementing an injection of carbon dioxide (CO2), even nitrogen, to produce petroleum.
Thus, in the next few years, carbon dioxide may prove to be a particularly advantageous gas, to the extent that it can be used by the producers for the assisted recovery of petroleum.
The technique for assisted recovery of petroleum by flushing with carbon dioxide (CO2) that is introduced into an injection well proves effective for reducing the viscosity of the petroleum in place and for increasing its mobility, which facilitates its recovery. Thus, a portion of the injected carbon dioxide is dissolved in the crude oil solution and can therefore be recovered with the petroleum during its production to be recycled later. A non-negligible portion of CO2 is sequestered in the formation, however. This phenomenon of CO2 sequestration thus participates in the reduction of CO2 emissions in the atmosphere that are responsible for the greenhouse effect and the heating of the planet.
For the producer, this advantage is rather a drawback since a portion of the injected CO2 is irretrievably lost. It therefore proves necessary to compensate for the losses of CO2 due to its sequestration in the formation by an addition of CO2, which of necessity carries a cost.
Furthermore, the CO2 stream that is recovered from a production well contains hydrocarbons. This gaseous and liquid stream that is under pressure is separated in gas-liquid separators, to obtain a liquid phase C3+ hydrocarbons, and a gaseous phase containing primarily CO2 and a substantial amount of impurities of methane with a little less ethane. These impurities can represent 5 to 12 mol % of the separated gaseous phase. In some cases where acidic crude oils are produced, the associated gas can also contain the hydrogen sulfide that is found in part as an impurity (several percent, for example).
Whereas one possibility would be to reinject this impure CO2 stream that contains methane and ethane into the formation, such action would deleteriously affect the saturation pressure of the formation. Other possibilities would be to mix impure CO2 stream with pure CO2 obtained from an outside source, or with other hydrocarbons that are heavier than the impurities in such a way as to dilute it and to counterbalance the volatility of the methane, but these technical solutions are very costly.
Cryogenic distillation could also be used to extract the methane and the ethane from CO2 and then the separated CO2 could be recycled into the formation. This separation, however, also proves to be very expensive.
Solvents exist that can absorb CO2, preferably with hydrocarbons, but these solvents would very easily absorb the hydrogen sulfide that is present, which would produce CO2 that is polluted by H2S.
U.S. Pat. No. 4,344,486 teaches hydrocarbons that contaminate carbon dioxide can be combusted by an oxygen-enriched gas or by essentially pure oxygen.
Nevertheless, there does not seem to exist any inexpensive separation technique that is sufficiently environmentally benign.
A first object of the invention is to separate the carbon dioxide from the hydrocarbon impurities that it contains.
A second object is to recycle this essentially pure carbon dioxide in an injection well so as to implement a process for assisted recovery of petroleum that is contained in a formation.
A third object is to carry out a combustion of these impurities and to recover the combustion heat.
Another object is to recover the combustion effluents so as to recycle them with the purified CO2 in a hydrocarbon injection well and thereby to reduce the emissions of CO2 and sulfur dioxide (SO2) into the atmosphere.
In a more detailed manner, the invention relates to a process for purification of a G/L two-phase effluent that contains carbon dioxide and gaseous and liquid hydrocarbons that are recovered from at least one hydrocarbon production well that is assisted by an injection of carbon dioxide (CO2), in which:
a) said pressurized G/L effluent is circulated at least once in at least one main gas-liquid separator, and C3+hydrocarbons and a gaseous effluent containing a major portion of carbon dioxide CO2 and a minor portion of methane and ethane are recovered,
whereby the process is characterized in that:
b) a pressurized combustion of the gaseous effluent is carried out in the presence of air in a combustion reactor, and a combustion effluent that is enriched with carbon dioxide and water vapor and that contains nitrogen is recovered,
c) said combustion effluent is cooled at least once to recover the heat,
d) the nitrogen is separated at least in part from the combustion effluent; and
e) the pressurized, cooled combustion effluent is recycled in an injection well using recycling means.
One of the important advantages of the process is to be able to reinject, within the framework of the implementation of the process for assisted recovery of petroleum, essentially pure carbon dioxide, that from which impurities were removed plus the one that is obtained from the combustion of said impurities. The cost of the addition of CO2 is thus decreased, the release of CO2 and optionally SO2 into the atmosphere are thus reduced by the same token, and the combustion energy of pollutants can be recovered.
According to a characteristic of the process, it is possible to control the increase in temperature of the combustion reactor due to the exothermicity of the combustion reaction by indirect heat exchange of the combustion effluent with a coolant.
According to another characteristic of the process, it is possible, after cooling stage (c), to separate the condensed water from the cooled combustion effluent in an H2O/CO2 separator.
According to a particularly advantageous first variant of the process, the oxygen-containing gas that is used as an oxygen carrier is air. It may be preferable, primarily in the presence of air, that the combustion reactor contain a catalyst when the hydrocarbons are in a very small quantity, for example less than 4%, especially as they will be diluted by nitrogen. The combustion temperature can then be lowered between 600 and 800xc2x0 C., for example, and the nitrogen oxide formation can be reduced.
According to a method of this first variant, the combustion effluent that is enriched with carbon dioxide and water vapor that results from the combustion in air of the gaseous effluent of stage (a) contains the nitrogen that is separated at least in part from the carbon dioxide after cooling stage (c) and preferably after the subsequent stage for separation of the condensed water.
The separation by cryogenic distillation of the nitrogen from the combustion effluent that essentially contains CO2 and N2 can be carried out more easily than that of nitrogen and oxygen from the air upstream from the combustion reactor since there exists a difference of boiling points of 117xc2x0 C. in the case of the N2xe2x80x94CO2 separation, whereas it is only 13xc2x0 C. in the case of the N2xe2x80x94O2 separation.
According to a second variant of the process, the oxygen-containing gas can be essentially pure oxygen, for example with less than 10% impurities. The formation of nitrogen oxides is then avoided. The combustion reactor can contain a catalyst.
According to a characteristic of this variant, it is possible to separate the nitrogen that is contained in the compressed air so as to recover essentially pure oxygen in a suitable separation unit that is placed upstream from the combustion reactor.
The G/L effluent that contains carbon dioxide and hydrocarbons that are obtained from the production well can contain hydrogen sulfide. The combustion of this gas in the reactor delivers SO2.
According to a first implementation, the sulfur dioxide that is produced can be recycled in the well with the carbon dioxide.
According to a second embodiment, the sulfur dioxide can be separated from the combustion effluent.
The separation stage of the water in the H2O/CO2 separator after the cooling of the combustion effluent can comprise a scrubbing stage that makes it possible to recover an effluent that contains water and SO2.
This scrubbing stage can comprise, moreover, a direct heat exchange between the combustion effluent that is cooled in part, and the cold water, which contributes to its final cooling.
A portion of the condensed water that was recovered in the H2O/CO2 separator can be cooled by indirect exchange and recycled at the top of the H2O/CO2 separator to carry out the scrubbing of the effluent that contains the SO2.
It is preferable to introduce the hydrocarbons and the carbon dioxide that emerge from the production well into a main gas-liquid separator. On the one hand, a first fraction that contains the C3+ liquid hydrocarbons is recovered at the bottom of said separator, and a second fraction that contains carbon dioxide and a minor portion of hydrocarbons is recovered at the top; said second fraction is compressed, and the compressed second fraction is circulated in a secondary gas-liquid separating means in such a way as to separate said gaseous effluent from the remaining C3+ hydrocarbons.
This secondary separating means can be a conventional separator or a cooling element coupled to a gas-liquid separator.
According to another modification, it may be advantageous to introduce an addition of fuel into the combustion reactor in such a way as to maintain the temperature level of the combustion reactor at an essentially constant level. The fuel may be the GPL that is obtained from the secondary separator, for example a flow that is high in carbon monoxide and/or hydrogen sulfide or a combustible gas that is obtained from another source.
The conditions of implementation of the process are generally as follows:
main combined gases/C3+ fraction separator
pressure of 1 to 20 bar, preferably 2 to 10 bar (1 bar=105 Pa)
temperature of 10 to 80xc2x0 C. and preferably 20-40xc2x0 C.
secondary C3+ gas/residual cold separator
pressure of 5 to 20 bar
temperature of xe2x88x9230 to 10xc2x0 C.
optionally catalytic combustion reactor
a) without catalyst and with oxygen, for example
O2/N2 separator by cryogenic distillation
pressure of 5 to 20 bar
temperature of 500 to 1000xc2x0 C.
or
b) with catalyst and air, for example
pressure of 3 to 8 bar
temperature of 450 to 800xc2x0 C., preferably 500 to 600xc2x0 C.
catalyst: extrudates or ceramic balls (alumina, for example, comprising at least one noble metal (Pt, Pd) or nickel
volumetric flow rate of 2,000 to 10,000 vol./vol./h
heat exchanger
temperature at the inlet of the reactor of 300 to 700xc2x0 C., preferably 450xc2x0 C. to 600xc2x0 C., for example
N2/CO2 separator by cryogenic distillation.